(1) Field of the Invention
The present invention relates to a rotorcraft control system that includes at least one control unit such as a cyclic stick, a collective pitch lever, or rudder pedals. Such a control unit makes it possible to modify the angular orientation of the blades of a rotorcraft rotor. More specifically, the collective stick and the pitch lever respectively control the collective pitch and the cyclic pitch of the blades of a main rotor of a rotorcraft, and the rudder, in particular, controls the collective pitch of the blades of an anti-torque tail rotor.
Furthermore, such a control unit is suitable for being operated either directly by a pilot or automatically, whether in a pilot cockpit or, when the rotorcraft is not carrying a pilot, from outside the rotorcraft.
(2) Description of Related Art
Accordingly, the rotor driven by the control system may be a main rotor that provides lift for the aircraft, or a tail rotor, such as an anti-torque rotor, that makes it possible to counteract the rotational yawing motion generated by a main rotor. Naturally, such a control system may also allow a main rotor and an anti-torque rotor of a rotorcraft to be driven simultaneously.
However, such a control system may also have other applications, and, in particular, may be applied to hybrid or combination rotorcraft, which are also designated by the English term “compound”, and to rotorcraft having two main rotors configured in tandem or two counter-rotating main rotors.
The invention also relates to a rotorcraft equipped with such a rotor control system and a rotor control method that is implemented by means of a control unit. The invention also relates more specifically to the function consisting of adapting the control of the rotorcraft during the takeoff and landing phases.
In point of fact, depending on the mission assigned to it, a rotorcraft may be called upon to operate from extremely diverse landing sites. Accordingly, the topography of the ground or of the landing sites may be extremely varied. In particular, such sites may be sloping or tilted; elevated, such as platforms; or mobile, such as the deck of a boat.
Their surface condition or level of preparation may also differ from one site to another. Thus, the landing site may take the form of a marked concrete runway, sandy terrain, or an area of unprepared ground.
Last, the direct environment of the landing sites may be more or less rich in terms of visual markers or references for the pilot. Under certain extreme circumstances, this environment may even significantly interfere with the pilot's vision, as in the case of so-called “dust landings” (also referred to in English as “brown-outs”), or in the case of landings on snow-covered terrain (referred to in English as “white-outs”).
Takeoff and landing strategies may also vary depending on the terrain and the mission. In particular, they may include purely vertical maneuvers or so-called “rolling” maneuvers.
Thus, the range of situations that the crews may face requires that the rotorcraft be equipped with a control system that is simultaneously robust, accurate, and fast during the complex phases consisting of landings and takeoffs.
These situations include, in particular, the operations that take place between flight and the fully landed state of the rotorcraft. This is typically the case when personnel such as physicians or rescue workers are disembarked on sloping terrain. This maneuver is typically performed by positioning the nose of the rotorcraft facing the rising terrain, with the forward landing gear in contact with the ground and with the rear landing gear off the ground. The individuals, who usually must climb down from or into the rotorcraft as quickly as possible, can then use the two side doors. Landings on sloping terrain are some of the most delicate maneuvers that rotorcraft crews are required to perform.
In point of fact, the control of a rotorcraft differs depending on whether the rotorcraft is in flight or in contact with the ground. Accordingly, different control laws are traditionally used to control a rotor, depending on whether the rotorcraft is or is not in contact with the ground. If the in-flight control law is retained when the rotorcraft is in contact with the ground, the preservation of the control goal would be negatively affected by this contact with the ground. The use of such a control law when the rotorcraft is in contact with the ground could, for example, cause the tilting of the rotor or even an amplification of ground resonance.
Therefore, it is customary to use a ground control law that is very different from the in-flight control law. Thus, the structure and the benefits of the ground law usually differ greatly from those of the in-flight control law. The in-flight control law favors the maintenance of flight parameters, whereas the goal of the ground control law consists of directly controlling the position of the rotor. Therefore, control of the helicopter must be adapted between these two operating modes, while maintaining a maximum level of assistance and the controllability of the aircraft during the transition phase between on-the-ground and in-flight situations. This adaptation of the control modes assumes the availability of information about the status of the aircraft in relation to the ground. This information is usually referred to collectively as the “ground/flight logic”.
The control laws for a rotorcraft also generally make use of an adjuster defined by a proportional gain and an integral time constant. Accordingly, the adjuster includes an integrator that makes it possible to ensure the long-term stability of a parameter or an objective, such as an attitude of the rotorcraft or the traveling speed of the rotorcraft.
Such integrators are described, in particular, in documents EP 2 672 357 A1 and WO 2008/108787 A2, in which the integral time constant is introduced into the control law by a command from a pilot assistance system, typically referred to in English as the “trim” box. Such a box makes it easier to keep the control unit in a given position. Meanwhile, the proportional gain contributes toward the short-term stability of the parameter. Thus, such control laws are typically used to steer the rotorcraft in flight.
However, when they are activated on the ground, the proportional gains and the integral time constants can have dangerous effects, as described in paragraph [0024] of the WO 2008/108787 A2 patent, such as the tilting of the helicopter on the ground, also referred to in English as “roll-over”.
In point of fact, when in contact with the ground, the rotorcraft no longer has the same degrees of freedom that it has when in flight. Consequently, the proportional gains and the integral time constants try to cancel an error that does not exist, in view of the stress applied to the rotorcraft by the ground. This residual error then causes a phenomenon known as control drift (also referred to as “swerving”), which can place the rotor in a position such that the force that it develops literally causes the rotorcraft to tilt.
Furthermore, the control laws that are appropriate for the “in flight” state of the rotorcraft usually entail a high level of proportional gain. These substantial gains are intended to ensure the stability of the rotorcraft, allowing the rejection of perturbations and a rapid response to instructions from the control unit.
Nevertheless, a high gain level leads to the appearance of a specific new risk on the ground, currently referred to as the “ground resonance” phenomenon. Pilots can train themselves to avoid this risk, for example, through the use of dedicated simulators, such as the one described in U.S. Pat. No. 3,346,969 A. The occurrence of ground resonance depends on the particular characteristics of the rotorcraft and of its control system, and on the particular nature of the landing site.
Furthermore, it is known that in order to avoid these risks, simpler control laws can be implemented, and the proportional gains of the control laws can be reduced when contact with the ground is detected.
Contact with the ground can be also identified through the use of dedicated sensors, often referred to in English as “weight-on-wheel” (WoW) or “weight-on-gear” (WoG) sensors, depending on their location on the landing gear. These sensors usually produce a discrete state, as described in document WO 2008/108787 A2, and, less commonly, a continuous state that makes it possible to define several distinct states of the landing gear, as described in document EP 2 672 357 A1.
Document EP 2 672 357 A1 also describes a simplification of the control laws when a rotorcraft is landing, by canceling the integral adjustment at the first sign of contact with the ground. Such a cancellation of the integral adjustment is referred to in this document by the use of the term “grounded” in paragraph [0068] to designate the state of the longitudinal integrating adjusters.
Such a state of the adjusters thus consists of freezing the output of the integrator, for example, by means of a null input. The integrator no longer operates, but retains the memory of the last command that was executed, thereby ensuring the continuity of the instruction sent to the servocontrols that control the rotor.
The error corresponding to the difference between the setpoint and the measurement passes through a gain control system and then through an adjuster, which includes at least one integrator, which can be placed in the so-called “grounded” state (that is, it can be rendered inoperative by means of a switch that sets its input to zero, depending on the ground/flight states).
When the integrator is rendered inoperative, the slaving of the measurement to the setpoint no longer takes place, and a static error can be stored on the adjusted control setpoint. Thus, the authority of the parameter is reduced, as is the authority of the associated command. For example, paragraph [0064] states that the control gain is reduced. Thus, the adjusted control setpoint at the output of the adjuster is immediately reduced accordingly, thereby also enabling a reduction in the control authority. For a given displacement of the control unit, the resulting command is then weaker.
Conversely, when an integral adjustment is retained via an active adjuster during this landing phase, the integrator performs an integration until the static error becomes null. Thus, the preservation of an active integrator ensures the preservation of authority. It should be noted that this is independent of the gain level. Even with a reduced control gain, the control setpoint will reach the same final level more slowly.
It should also be noted that the preservation of the authority allocated to the pilot is of primordial importance for landings on sloping terrain, during which stabilized controls are often reached near the mechanical stops of the actuators.
Thus, the present invention and document EP 2 672 357 A1 reflect major differences in implementation and in terms of their goals.
It should be noted that the present invention can easily be generalized to mechanical flight controls, which is obviously not the case with the teachings of document EP 2 672 357 A1. Accordingly, the invention is particularly well suited to automatic landings and takeoffs.
Furthermore, even if the control systems described in the prior art make it possible to guard against certain risks that are specific to missions involving rotorcraft on the ground, these control systems are not satisfactory in terms of controlling the rotorcraft.
Indeed, on the one hand, such control systems entail a significant change in rotorcraft piloting philosophy during a delicate phase of operations. For the pilot, this change in philosophy is accompanied by an additional workload, with a non-negligible risk of pilot-induced oscillation (PIO). On the other hand, such control systems are oriented toward standard landing and takeoff maneuvers rather than toward missions involving intermediate states of the rotorcraft between flight and the ground.
Accordingly, a crew performing a landing on sloping terrain may find itself without assistance during the period between touchdown and the fully landed state of the rotorcraft, in a maneuver that is still extremely delicate. Consequently, such control systems are not well suited to the disembarkation of individuals from a rotorcraft that is positioned with its nose facing the rising terrain.